Temporal and pathogen-load dependent changes in rainbow trout (Oncorhynchus mykiss) immune response traits following challenge with biotype 2 Yersinia ruckeri

Rainbow trout infected with Yersinia ruckeri, the causative agent of enteric redmouth disease (ERM), produce a pro-inflammatory and acute-phase response attributed in part to the innate recognition of bacterial-produced flagellin. Recently, variants of Y. ruckeri have been identified that lack flagella and associated motility. These strains are classified as biotype 2 (BT2) Y. ruckeri and these are considered an emerging problem in salmonid aquaculture. Little is known about the salmonid immune response to these variants. Herein, we report temporal and quantitative changes in rainbow trout immune response parameters following a primary challenge with BT2 Y. ruckeri strain YRNC10. Fish were injection-challenged with ten-fold dilutions of viable bacteria and sampled on days 1, 3, 5 and 7 post-challenge. TNFalpha1 and IL1-beta1 transcripts were increased by day 1 post-challenge, and on days 3, 5 and 7 maximal gene transcript up-regulation occurred at a threshold of approximately 64-256CFU per mg spleen tissue. Infection induced robust SAA gene up-regulation that was significantly correlated with increased gene expression of IL-1beta1 (r=0.81, P